1. Field of the Invention
This invention relates to a method of polarizing high molecular weight film. More specifically, it relates to a method of polarization for polarizing high molecular weight film in which polarization is effected on the high molecular film wound in a specific manner around a core such as a roll, plate or the like, thereby producing a large quantity of polarized film at one time with excellent efficiency.
2. Description of the Prior Art
Methods of polarizing high molecular weight film such as polyvinylidene fluoride, polyvinyl fluoride, and the like under a static high electric field to produce piezoelectric devices, etc., are well-known. One conventional method for effecting this polarization comprises laying, while stretching, film to be polarized as plain sheets and applying a high static electric field at a prescribed temperature on the film using thin films of vapor-deposited aluminum or an electroconductive paint and the like as electrodes provided on both surfaces of the film.
The method has defects such as poor workability in the polarization, low production efficiency and the like. Since the polarization is generally effected in a heating oven, film to be polarized should be cut into a size which will fit into the heating oven (usually less than several hundred square millimeters). In addition, highly stretched film is usually used with the aim of increasing the piezoelectric and pyroelectric effects and such film must be supported by a film frame uniformly and without any slackening, which is a very difficult operation. Moreover, even if the polarization can be attained in such a way, the amount of film that can be obtained at one time is only on the order of 10 to 20 sheets with a width of 30 cm, a length of 30 cm, considering the thickness of the frame for supporting the film and using a heating oven of a volume of 90 1 (45 cm .times. 45 cm .times. 45 cm).
Further, in such a polarization method the electrode-forming portions, e.g., vapor deposited aluminum, must be removed along the periphery of the film in cutting and stacking the film, and additional film area must be provided for attaching the film to the supporting frame. This results in a reduction in the yields of the polarized film and production efficiency.